This invention relates to driving methods for liquid crystal displays wherein a layer of liquid crystal material has voltages selectively applied thereacross to render said regions selectively visually identifiable. Such liquid crystal displays are particularly suitable as numeric or alpha-numeric displays for electronic table calculators, electronic timepieces, measuring instruments and the like due to the fact that such displays can be operated at relatively low power, can be formed in an extremely thin configuration, and otherwise offer packaging, cost and manufacturing advantages.
The methods of driving liquid crystal displays may be classified into two classes, static drives and dynamic drives. In the static drive method, each digit of the display is provided with a set of segmented electrodes necessary to define the characters to be displayed, as, by way of example, the seven segments of a seven-bar display utilizable in a numeric display. Each digit would also be provided with a common electrode. Separate driving circuitry would be provided for each digit, so that each digit of the display would be separately and simultaneously energized. In effect, each digit is controlled independently by data signals applied to the associated driving circuitry.
On the other hand, in the dynamic drive method, time division multiplexing is utilized so that each segment or digit is sequentially energized. In the case of a multifigure liquid crystal display formed from a plurality of seven-bar arrays of segmented electrodes, corresponding segments of each digit would be electrically coupled together and a single driving circuit would be applied to the commonly connected segmented electrodes. A time-division timing signal is sequentially applied to each common electrode of each digit, the data signal applied to the segmented electrodes rendering visible the appropriate segments of the digit having an energized common electrode. In effect, each of the digits is rendered momentarily visible in sequence at a relatively high frequency, the display giving the appearance of continuous operation due to retinal retention. The above-described display could be considered an X-Y matrix-type display. Still another matrix-type display is a dot-matrix display defined by the intersection of a plurality of parallel row electrodes and a corresponding plurality of parallel column electrodes, the column electrodes extending substantially at right angles to the row electrodes, the liquid crystal material being supported between the column and row electrodes. The intersection of a column and a row electrode represents a point or dot on the matrix, the selected energization of groups of such points or dots providing an alpha-numeric display of a single digit or a plurality of digits if a sufficient number of column electrodes are provided. The row electrodes of each digit may be electrically connected together and driven by a single driver with a time-division timing signal applied to the column electrodes to sequentially energize each such column electrode.
The dynamic drive method is advantageous in that both design and manufacture expense is substantially reduced through the substantial reduction in input signal lines to the display elements, as well as reduced driving circuitry. However, the dynamic drive method produces an inferior display effect as compared to that of the static drive method due to the poor contrast between regions of the liquid crystal material to be rendered visually identifiable and regions which are not to be rendered visually identifiable caused by the periodic application of the time-division signals to each segment during the duty cycle of the dynamic drive method.
By determining the applied voltage for each segment in accordance with the duty cycle, an optimum display effect may be produced in liquid crystal displays driven by the dynamic drive method.